feat(backend): T1.3 月报管理 CRUD — 列表/详情/创建/更新/提交/删除

- 路由：GET/POST/PUT/DELETE /api/v1/reports + POST /reports/{id}/submit
- 状态机：draft → submitted → ai_parsed → reviewed
- 防重复：同年同月同企业不可重复创建
- 租户隔离：通过 company 关联校验
- 测试：7 个月报测试（总计 33 tests passed）

backend/app/main.py

@@ -12,6 +12,7 @@ from fastapi.responses import JSONResponse
 
 from app.routers.auth import router as auth_router
 from app.routers.companies import router as companies_router
+from app.routers.reports import router as reports_router
 from app.schemas.common import error
 
 
@@ -69,3 +70,4 @@ async def health_check():
 
 app.include_router(auth_router, prefix="/api/v1")
 app.include_router(companies_router, prefix="/api/v1")
+app.include_router(reports_router, prefix="/api/v1")